Process for the production of aryl carboxylic acids

ABSTRACT

Aryl carboxylic acids are produced from alkyl substituted aryl compounds or partially oxidized alkyl substituted aryl compounds by the oxidative reaction of the corresponding alkyl substituted aryl compounds, partially oxidized alkyl substituted aryl compounds or mixtures thereof with an alkali metal dichromate in aqueous solution to form an alkali metal aryl carboxylate and chromic oxide, separation of the reaction products, oxidation of the chromic oxide to chromium trioxide in an electrolytic cell, reaction of the chromium trioxide with the alkali metal aryl carboxylate to form the corresponding aryl carboxylic acid and an alkali metal dichromate. The oxidation of p-toluic acid to terephthalic acid is an illustrative example of the process of invention. Other aryl carboxylic acids which can be produced by this process include benzoic acid, trimellitic acid, and 2,6naphthalene dicarboxylic acid. Benzoic acid is useful as a preservative. The aryl di- and tri-carboxylic acids are useful in the production of polyesters.

United States Patent [1 1 Engenbrecht, deceased et al.

11] 3,726,914 51 Apr. 10, 1973 [54] PROCESS FOR THE PRODUCTION OF ARYL CARBOXYLIC ACIDS [75] Inventors: Robert M. Engenbrecht, deceased, late of St. Louis, M0. by Alice M. Engelbrecht, executrix; James C. Hill, Chesterfield; Richard N. Moore, St. Louis, both of Mo.

[73] Assignee: Monsanto Company, St. Louis, Mo.

[22] Filed: June 29, 1970 [21] Appl. No.: 50,981

Primary Examinerl-lenry R. Jiles Assistant ExaminerR. S. Weissberg Attorney-John L. Young, Richard W. Sternberg, Paul L. Passley and Neal E. Willis [5 7] ABSTRACT Aryl carboxylic acids are produced from alky] substituted aryl compounds or partially oxidized alkyl substituted aryl compounds by the oxidative reaction of the corresponding alkyl substituted aryl compounds, partially oxidized alkyl substituted aryl compounds or mixtures thereof with an alkali metal dichromate in aqueous solution to form an alkali metal aryl carboxylate and chromic oxide, separation of the reaction products, oxidation of the chromic oxide to chromium trioxide in an electrolytic cell, reaction of the chromium trioxide with the alkali metal aryl carboxylate to form the corresponding aryl carboxylic acid and an alkali metal dichromate. The oxidation of p-toluic acid to terephthalic acid is an illustrative example of the process of invention. Other aryl carboxylic acids which can be produced by this process include benzoic acid, trimellitic acid, and 2,6- naphthalene dicarboxylic acid. Benzoic acid is useful as a preservative. The aryl diand tri-carboxylic acids are useful in the production of polyesters.

PROCESS FOR THE PRODUCTION OF ARYL CARBOXYLIC ACIDS FIELD OF THE INVENTION The present invention relates to the preparation of aryl carboxylic acids. More particularly it relates to the preparation of aryl carboxylic acids by the oxidation of the corresponding alkyl substituted aryl compounds and partially oxidized alkyl substituted aryl compounds.

DESCRIPTION OF THE PRIOR ART It has been recognized that sodium dichromate is an I effective oxidizing agent. U. S. Pat. No. 3,330,862 teaches the oxidation of dimethylnaphthalene to 2,6- naphthalene dicarboxylic acid by sodium dichromate. Previously, U.S. Pat. No. 2,792,420 had commented that oxidation in aqueous solution with alkali metal bichromate, under pressure and at elevated temperature had found application on an industrial scale for the oxidation of, for example, toluene. The disadvantage of the oxidation of aralkyl hydrocarbons with alkali metal bichromates in aqueous solution is the large amount of chromium oxide (Cr O produced, which for reasons of economy must either be recovered as pigment or reconverted into bichromate for reuse. Since onemole of chromium oxide is produced per methylene group oxidized, the oxidation of aralkyl hydrocarbons having several or relatively long side chains is technically unattractive because of the large amounts of chromium oxide to be handled.

As a result of the recognized difficulty of obtaining reasonable value for the chromium oxide by-product or regenerating it to its useful oxidizing state, little use has been found for alkali metal dichromate as an oxidizing agent in the preparation of aryl carboxylic acids. Electrolytic processes for regenerating the dichromate salt have been suggested but have not been of general commercial significance because of low yields and the substantial cell investment required. Thus, a process which would utilize alkali metal dichromate as an oxidant for alkyl substituted aryl compounds and partially oxidized alkyl substituted aryl compounds to aryl carboxylic acids and efficiently regenerate the alkali metal dichromate to its original oxidation state would fill this recognized industry need and advance the state of the art.

SUMMARY OF THE INVENTION The process of this invention for the preparation of aryl carboxylic acids from the corresponding alkyl substituted aryl compounds and partially oxidized alkyl substituted aryl compounds comprises charging to a reactor a feed stock selected from the group which consists of an alkyl substituted aryl compound, a partially oxidized alkyl substituted aryl compound and a mixture of an alkyl substituted aryl compound and a partially oxidized alkyl substituted aryl compound, mixing with said feed stock an aqueous solution of an alkali metal dichromate, reacting said mixture at a temperature above 150 Centigrade whereby said feed stock is oxidized to the corresponding alkali metal aryl carboxylate and said alkali metal dichromate is reduced to chromic oxide, separating said chromic oxide from the reacted mixture, dissolving said chromic oxide ina strong mineral acid, charging the dissolved chromic oxide to an electrolytic cell, passing a direct electric current through said cell whereby the chromic oxide is oxidized to chromium trioxide, separating the chromium trioxide from the strong mineral acid solution, mixing the chromium trioxide with an aqueous solution of alkali metal aryl carboxylate, reacting said chromium trioxide with said alkali metal aryl carboxylate whereby the corresponding aryl carboxylic acid and alkali metal dichromate in aqueous solution are formed and separating said aryl carboxylic acid from the aqueous alkali metal dichromate solution.

DETAILED DESCRIPTION Alkyl substituted aryl compounds useful as a feed stock in the process of this invention may contain one aromatic nucleus or may be polynuclear;.e.g., benzene, naphthalene, anthracene, phenanthrene, diphenyl, triphenyl, and the like, having aliphatic substituents. The chain length of the aliphatic substituents is not critical. Alkyl groups having as many as eight to ten carbon atoms in the chain may be used. However, for economy, ease of product, purification, and handling considerations it is preferable to limit alkyl group chain lengths to six carbon atoms or less. Still more preferred are aryl compounds with aliphatic substituents having a chain length of four carbon atoms or fewer. Examples of typical preferred alkyl substituted aryl compounds include methylbenzene, ethylbenzene, n-propylbenzene, i-propylbenzene, n-butylbenzene, s-butylcyclohexylbenzene, dimethylbenzene, diethylbenzene, dim-propylbenzene, di-i-propylbenzene, di-n-butylbenzene, di-s-butylbenzene, trimethylbenzene, triethylbenzene, tri-n-propylbenzene, tri-i-propylbenzene, tri-n-butylbenzene, tri-sbutylbenzene, ethyltoluene, ethyl-n-propyltoluene, ethyl-i-propyltoluene, ethyl-s-butyltoluene, diethyltoluene, diethyl-n-propyltoluene, diethyl-i-propyltoluene, diethyl-s-butyltoluene, triethyltoluene, triethyl-n-propyltoluene, triethyl-i-propyltoluene, triethyl-s-butyltoluene, n-propylethylbenzene, ipropylethylbenzene, n-butylethylbenzene, s-butylethylbenzene, dimethylethylbenzene, di-n-propylethylbenzene, di-i-propylethylbenzene, di-n-butylethylbenzene, di-s-butylethylbenzene, trimethylethylbenzene, tetraethylbenzene, .tri-n-propylethylbenzene, tri-i-propylethylbenzene, trim-butylethylbenzene, tri-sbutylethylbenzene, i-propyl-n-propylbenzene, n-butyln-propylbenzene, s-butyl-n-propylbenzene, dimethyl-npropylbenzene, di-i-propyl-n-propylbenzene, di-nbutyl-n-propylbenzene, di-s-butyl-n-propylbenzene, trimethyl-n-propylbenzene, triethyl-n-propylbenzene, tri-n-propylbenzene, tri-i-propylbenzene, tri-n-butyl-npropylbenzene, tri-s-butyl-n-propylbenzene, n-butyl-ipropylbenzene, s-butyl-i-propylbenzene, dimethyl-ipropylbenzene, diethyl-i-propylbenzene, di-i-propylbenzene, di-n-propyl-i-propylbenzene, di-s-butyl-i-propylbenzene, trimethyl-i-propylbenzene, triethyl-i-propylbenzene, tri-i-propylbenzene, tri-n-butyl-i-propylbenzene, i-butyl-n-butylbenzene, dimethyl-i-butylbenzene, diethyl-i-butylbenzene, di-n-propyl-i-butylbenzene, di-i-propyl-i-butylbenzene, di-n-butyl-i-butylbenzene, di-s-butyl-i-butylbenzene, trimethyl-i-butylbenzene, triethyl-i-butylbenzene, tri-n-propyl-i-butylbenzene, tri-i-propyl-i-butylbenzene, dimethyl-s-butylbenzene, diethyl-s-butylbenzene, di-n-propyl-s-butylbenzene,

benzene, di-i-propyl-s-butylbenzene, di-n-butyl-s-butylbenzene, trimethyles-butylbenzene, triethyl-s-butylbenzene, tri-n-propyl-s butylbenzene, tri-i-propyl-s-butylbenzene, tri-n-butyl-s-butylbenzene, and correspondingly (or higher) substituted polynuclear materials; also alpha-methyl naphthalene, beta-methyl naphthalene and other higher substituted polynuclear materials. Alkyl substituted aryl compounds are also useful as a feed stock in mixtures of two or more alkyl substituted aryl compounds giving as a product a mixture of two or more of the corresponding aryl carboxylic acids.

Partially oxidized alkyl substituted aryl compounds useful as a feed stock in the process of this invention are partial oxidation products wherein the aliphatic substituents are converted to intermediate oxygenated derivatives such as alcohols, aldehydes, ketones, peroxide type compounds, carboxy acids having at least one substituted group on the aromatic nucleus capable of being oxidized to a carboxyl group, etc. Partially oxidized alkyl substituted aryl compounds are useful as a feed stock in mixtures of two or more of such partially oxidized feed stocks. Said mixtures may additionally contain one or more alkyl substituted aryl compounds and be useful as feed stocks in the process of the present invention.

Illustrative examples of said partially oxidized feed stocks include, but are not limited to, aromatic compounds having non-oxidizable substituent groups besides at least one oxidizable methyl group such as mtoluic acid; p-toluic acid, m-methyl toluate and pmethyl toluate; hydroxymethyhsubstituted aromatic compounds such as benzyl alcohol, mand p-hydroxymethyl toluenes, and tolyl carbinols; aldehyde-substituted aromatic compounds such as benzaldehyde, mand p-tolualdehydes, isophthaloaldehyde, terephthaloaldehyde, mand p-carboxy benzaldehyde and esters thereof; and those hydroxymethyl-substituted and aldehyde substituted aromatic compounds containing other non-oxidizable substituent groups. Non-oxidizable substituents consist of those groups which are not oxidized to carboxyl groups under the reaction conditions employed in this invention, such as a halogen atom, carboxyl group, carboalkoxy group, a cyano group and a nitro group. Other examples of suitable feed stocks for the present process are dimethyl biphenyl, methyl acetophenones, and the like.

The feed stock for the process of this invention may be a mixture of isomeric materials or such a mixture containing lower or higher homologues. It may also contain some saturated aliphatic hydrocarbon materials of similar boiling ranges. Mixtures of materials may be converted to the corresponding mixtures of aromatic carboxylic acids, which acids may then be separated, e.g., by physical means such as distillation, or by a' combination of chemical and physical means such as esterification followed by fractionation.

A preferred feed stock has the formula R Ar coon wherein Ar is an aryl group, R is selected from the group which consists of an alkyl group, an aldehyde group, a carbinol group and a ketone group, m is a number from one to four, n is a number from zero to three, m n is a number from one to four and when m is a number greater than one the Rs need not be the same group.

A feed stock of xylenes and partially oxidized xylenes, such as toluic acids, aldehydes and ketones is particularly useful in the production of terephthalic acid by the process of this invention. The ortho and meta phthalic acids thus produced may be isomerized to the terephthalic acid by known methods. The oxidation of p-toluic acid to terephthalic acid, which is a difficult oxidation by conventional methods, is readily accomplished by the process of this invention. Although feed stocks useful herein may be soluble in the aqueous alkali metal dichromate solution, the oxidation of the feed stock takes place without the necessity for the feed stock being soluble in the aqueous alkali metal dichromate solution. Preferred feed stocks are substantially insoluble in the aqueous alkali metal dichromate solution.

In a preferred embodiment of the process of this invention wherein the desired product is terephthalic acid, mixed xylenes are oxidized to the corresponding toluic acid by a known method, such as the cobalt catalyzed oxidation by air. The toluic acids may be oxidized to the corresponding phthalic acids by the process of this invention and then the resulting mixed phthalic acids may be isomerized to terephthalic acid by a known method such as is taught in US. Pat. No. 2,863,913. Under certain circumstances it may be advantageous to convert the resultant mixture of toluic acids to paratoluic acid before proceeding with the second oxidation. In this embodiment of the invention, mixed ortho and meta toluic acids are isomerized to para toluic acid by heating salts of said acids to from about C to about 450 C in a reactor under an inert atmosphere such as carbon dioxide at a pressure ranging from about atmospheric pressure to about 2,000 pounds per square inch gauge in the presence of an alkaline earth metalhalide catalyst such as cadmium iodide, a basic metal carbonate and a desiccant such as aluminum carbide. The resulting para-toluic acid is then oxidized to terephthalic acid.

Alkali metals are metals in the first group of the Periodic Table, i.e., lithium, sodium, potassium, rubidium and cesium. Preferred alkali metals are lithium, sodium and potassium. More preferred alkali metals are sodium and potassium. Where reactor size is desired to be minimized, potassium is still more preferred since the solubility of dipotassium terephthalate in water is higher than that of the other alkali metal terephthalates under the same conditions.

The reactions which take place in the process of this invention are illustrated by the oxidation of para-toluic acid to teriphthalic acid by potassium dichromate as described in the following equations:

water xooo-Q-ooox 2cm; H2O

nooc-Q-coorr K2CrzO1 The chemical oxidation of the feed stock by an alkali metal dichromate, as illustrated by Equation (1), is carried out at an elevated temperature, at least above 150 Centigrade in an aqueous medium. The reaction temperature may be varied over a wide range depending on the feed stock used, the desired rate of reaction, and other considerations. The pressure under which the reaction is carried out is not critical. However, in order to minimize evaporation losses during the reaction, it is necessary to conduct the reaction in a closed system under autogenous pressure. In a preferred embodiment the reaction temperature is maintained from about 175 Centigrade to about 400 Centigrade. More preferred is a temperature range of from about 200 C to about 350 C. Still more preferred is a temperature range of from about 225 C to about 300 C.

The electrolytic oxidation of chromium, as illustrated by Equation (2), is carried out in an electrolytic cell. The size, shape and dimensions of the electrolytic cell as well as the materials from which it is fabricated are not critical for the operability of the chromium regeneration step. Since a strong mineral acid is used in the cell, it is preferable that the surfaces of the cell exposed to the mineral acid be resistant to attack by the acid in order to obtain longer cell service life and minimize maintenance requirements. A cell with both a lead cathode and a lead anode has been found satisfactory. A carbon cathode is also acceptable as well as a platinum-plated steel cathode.

The current density of the cell is not critical to the chemistry of the process. A high current density is desirable for economic reasons. The amount of anode surface area required for a fixed electrolytic capacity is inversely proportional to the current density. A preferred embodiment of the process of the present invention is particularly advantageous in that current densities of over 40 amperes per square decimeter of anode surface area are achievable.

In this preferred embodiment high current densities are achieved by promotion of turbulent flow across the surface of the anode. Such turbulent flow minimizes stagnation at the anodic surface. Stagnation of higher oxidation state ions at the anodic surface leads to inefficient operation of the electrolytic cell. Thus, in this embodiment, chromium ions in the +3 oxidation state are rapidly transported to the anode where they are oxidized to the +6 oxidation state and immediately swept away by the turbulent flow. This immediate removal of +6 chromium ions from the anode surface permits additional +3 chromium ions to reach the surface of the anode where they in turn are oxidized and swept away. In this manner highly efficient cell operation is achieved. A highly efficient cell employing turbulent flow across the anode will oxidize a greater volume of ions than a normal cell. This means that for a given volume of ions to be oxidized fewer high efficiency cells are required than normal cells. This reduces not only the initial capital investment in the electrolytic cells but also minimizes the plant space required as well as the inventory of chromium and acid.

Turbulent flow may be achieved by high velocity flow and close electrode spacing. Maximum flow velocity is limited only by the structural strength of cell components and pumping capacity. Electrode spacing will depend on the size, type and configuration of the cell components. In suitable cells the anode may be spaced less than one-fourth inch from the cathode. In other cells achieving current densities of 40 amperes per square decimeter of anode surface area or more the spacing may be greater than one-half inch. Neither the spacing nor the flow rate of itself is critical. The key consideration is to maintain turbulent flow across the surface of the anode and minimize stagnation of +6 chromium ions at the anodic surface.

Another key factor in achieving high efficiency in cell operation is minimizing the flow of +6 chromium ions to the cathode where they would be reduced to a lower oxidation state. This can be achieved by a number of procedures. The cell may be designed so that the direction of high velocity flow is always away from the cathode and toward the anode. A porous anode and cathode may be useful in such designs. Another method of preventing the +6 chromium ions from migrating to the cathode is to erect a barrier between the anode and cathode. Such a barrier can be a porous charged conductive shield to repel the ions as they try to approach the cathode. A more effective barrier is the use of a porous membrane to separate the anolyte from the catholyte. The porous membrane separates the electrolytic cell into two compartments, one of which compartments contains the anode and the second of which contains the cathode. If the cell contents, the electrolyte, is to pass from one compartment to the other, it must pass through the porous membrane. The membrane is a more effective barrier where the catholyte is maintained at a higher pressure than the anolyte. Such a pressure differential may be easily maintained by charging the chromic oxide to the compartment containing the cathode and withdrawing the chromium trioxide solution from the compartment containing the anode at about equivalent rates. In this manner, any flow through the membrane is directed away from the cathode and toward the anode. One of several satisfactory porous membranes, which are resistant to a strongly acid environment, is a cloth of polytetrafluoroethylene.

Preferred strong mineral acids for use in the chromium regeneration step are sulfuric acid and phosphoric acid. In still more preferred embodiments sulfuric acid is found to give very good results. The acid concentration is not critical so long as the chromium remains in solution during the course of the electrolytic oxidation process. Acid concentrations of about 40 to percent are conveniently used. I-Iigher acid concentrations provide high electrical conductivity and minimize the amount of water to be removed in the recovery of chromic acid from the anolyte. The acid must not be so concentrated as to cause precipitation of chromic acid in the electrolytic cell.

The acidification of the alkali metal aryl carboxylate with chromium trioxide as illustrated by Equation (3) is conducted in aqueous solution. Surprisingly, the reaction readily produces the corresponding aryl carboxylic acid and the alkali metal dichromate under ambient room temperature and normal atmospheric pressure.

. The several separation steps are by known techniques. Mixing during the chemical oxidation of the feed stock may be accomplished by any suitable means or technique so long as the reactants are sufficiently admixed to allow the chemical oxidation reaction to proceed. For example, for bench scale experimental work sufficient admixture may be achieved by a portable electric laboratory mixer at low speed and for a plant process a reactor with single anchor type rotor is satisfactory.

The process of this invention utilizes efficiently all energy inputs and minimizes by-product production. The closed circuit of related reactions conserves the most valuable reactants and eliminates by-product disposal problems. Acidification of the alkali metal aryl carboxylate with chromium trioxide is a key element in this process in that the production of alkali metal sulfate or phosphate is eliminated. Prior to the present invention it was thought that strong mineral acids were necessary to convert the alkali metal aryl carboxylates to the corresponding aryl carboxylic acid. The successful use of chromium trioxide in this acidification provides a unique approach which closes the gap in existing technology and along with the improved features of the other steps described herein makes possible the novel process of the present invention.

The following examples more specifically illustrate some of the preferred embodiments of this invention.

EXAMPLE 1 This example illustrates the oxidation of an alkyl substituted aryl compound by an alkali metal dichromate to form an alkali metal aryl carboxylate and the subsequent acidification of the alkali metal aryl carboxylate by an aqueous solution of chromium trioxide to produce an aryl carboxylicacid.

To a 2 liter stainless steel, stirred autoclave are charged 79.80 grams (0.752 moles) of p-xylene, 672.0 grams (2.255 moles) of Na Cr O '2l-l O and 700 grams (38.89 moles) of water. The reactants are heated, with stirring, to 250 C, and held in the range of 234250 C for 2 hours. The autoclave and contents are cooled rapidly to about 70 C; then the reaction mixture is discharged into a filter. The green, chromic oxide filter cake is washed with 1.9 liters of hot water.

The orange filtrate containing disodium terephthalate and the excess sodiumdichromate is added, at the rate of milliliters/minute to a stirred solution of chromic acid (300 grams chromiumtrioxide in 1.2 liters of water). The precipitated terephthalic acid is collected on a filter and washed until no color is detected in the wash water. The filtrate (sodium dichromate solution) can be concentrated to a desired level and used for another oxidation as illustrated in Example 19.

The crude terephthalic acid is found to contain 193 ppm chromium and 1.8 ppm iron. This is lowered to 110 ppm chromium and 1.5 ppm iron by slurrying twice with one liter portions of boiling water. Recrystallization from water (13 parts water to 1 part terephthalic acid) lowers the heavy metals content to 2.3 ppm chromium and 1.3 ppm iron.

EXAMPLE 2 This example illustrates the oxidation of a partially oxidized alkyl substituted aryl compound by an alkali metal dichromate to form an alkali metal aryl carboxylate and the subsequent acidification of the alkali metal aryl carboxylate by an aqueous solution of chromium trioxide to produce an aryl carboxylic acid.

245.89 grams (0.825 moles) Na Cr O '2l-I O, 102.11 grams (0.750 moles) p-toluic acid, and 700 grams (38.89 moles) water are charged to a 2 liter, stainless steel, stirred autoclave. The autoclave and its contents are heated to 240 C and held, with stirring, between 239 244 C for 30 minutes. The autoclave is cooled rapidly to 90 C and the reaction mass is discharged into a filter. The green, chromic oxide filter cake is washed with 1 liter of hot water.

The orange filtrate containing disodium terephthalate and excess sodium dichromate is added to a stirred solution of chromic acid 157.5 grams chromium trioxide in 300 milliliters of water). The precipitated terephthalic acid is collected on a filter and washed with two liters of hot water. The terephthalic acid after drying is found to weigh 112.93 grams (0.680 moles) which corresponds to a yield of 90.7 weight percent.

EXAMPLE 3 This example illustrates the oxidation of another partially oxidized alkyl substituted aryl compound by an alkali metal dichromate to form an alkali metal aryl carboxylate and the subsequent acidification of the al- ,kali metal aryl carboxylate by an aqueous solution of ture spontaneously increases to 239 C in 7 minutes; for

the 2 hour reactor residence time a temperature of 225 C is maintained. The reactor is next cooled rapidly to C and the reaction mixture is discharged into a filter. The green, chromic oxide filter cake is washed with 500 milliliters of hot water.

The orange filtrate containing disodium terephthalate excess sodium dichromate is slowly added to a stirred solution of chromic acid (30 grams chromium trioxide in 400 milliliters water). The precipitated terephthalic acid was collected on a filter and washed with water until no color was observed in the washings. The filter cake is slurried twice in boiling water. The terephthalic acid after drying is found to weigh 15.89 grams (0.0957 moles) corresponding to a 97.5 percent yield.

The sodium dichromate filtrate from the acidification step is recovered for a subsequent oxidation as illustrated in Example 19.

EXAMPLE 4 This example illustrates the oxidation of another alkyl substituted aryl compound by an alkali metal dichromate to form an alkali metal aryl carboxylate and the subsequent acidification of the alkali metal aryl carboxylate by an aqueous solution of chromium trioxide to produce an aryl carboxylic acid.

10.89 grams (0.1026 moles) m-xylene, 89.4 grams (0.2 moles) Na Cr O '2I-l O and 400 grams (22.2 moles) water are added to a 1.4 liter, stainless steel, rocking autoclave reactor. The reactor is sealed and heated to 275 C; the rocking reactor is held at this temperature for two hours. 'At the end of this time the reactor is cooled and the reactants are filtered. 0.0041 moles of m-xylene are recovered.

The orange filtrate containing disodium isophthalate and the excess sodium dichromate is acidified with chromic acid (30 grams of chromium trioxide in 400 milliliters of water). The precipitated isophthalic acid is filtered and washed with water until the washings are colorless. After drying the acid is analyzed and is found to contain 15.05 grams (0.0906 moles) isophthalic acid and 0.22 grams (0.0016 moles) m-toluic acid. This corresponds to a 93.5 percent yield of isophthalic acid.

EXAMPLE This example illustrates the oxidation of still another alkyl substituted aryl compound by an alkali metal dichromate to form an alkali metal aryl carboxylate and the subsequent acidification of the alkali metal aryl carboxylate by an aqueous solution of chromium trioxide to produce an aryl carboxylic acid.

13.47 grams (0.1004 moles) p-cymene, 149.0 grams (0.5 moles) Na Cr O '2H O, and 400 grams (22.2 moles) water are charged to a 1.4 liter, stainless steel, rocking autoclave reactor. The reactor is sealed and taken to 275 C; the rocking reactor is held at this temperature for 2 hours. At the end of this time the reactor is cooled and the reactants are filtered. 0.0046 moles of p-cymene are recovered.

The orange filtrate containing disodium terephthalate the excess sodium dichromate is acidified with chromic acid (30 grams of chromium trioxide in 400 milliliters of water). The precipitated terephthalic acid is filtered and washed with water until the washings are colorless. The terephthalic acid after drying is found to weigh 13.92 grams (0.0838 moles); this corresponds to a yield of 87.5 percent.

EXAMPLES 6 TO 13 These examples illustrate the wide variety of aryl carboxy acids which may be produced by the process of this invention from alkyl substituted aryl compounds and partially oxidized alkyl substituted aryl compounds.

The method of Example 5 is followed using the feed stocks shown in the examples to give a measurable quantity of the corresponding acids.

Example Feed Stock Acid 6 benzaldehyde benzoic acid 7 4-methyl-propiophenone terephthalic acid 8 4-4dimethyl bibenzyl terephthalic acid 9 mixed xylenes mixed phthalic acids 10 pseudo cumene trimellitic acid 1 l l,2,4,5-tetramethyl l,2,4,5-benzene tetrabenzene carboxylic acid 12 phenyl carbinol benzoic acid 13 2,6-dimethyl naphthalene 2,6-naphthalene discarboxylic acid The utility of the aryl carboxy acids which may be made according to the process of this invention is well known. The isomerization of o-phthalic acid and mphthalic acid to terephthalic acid is taught in U.S. Pat. Nos. 2,863,913, 2,863,914, 2,905,709 and 2,906,774. Benzoic acid and its salt, sodium benzoate are most widely used as preservatives agents for foods, pharmaceutical preparations and cosmetics. The largest use of terephthalic acid is in the production of poly(ethylene terephthalate) film and fibers. O-phthalic and m-phthalic acids may be used in the production of phthalate' ester plasticizers for polyvinyl chloride composition as well as for isomerization to terephthalic acid. Trimellitic acid likewise is used in the production of polyvinyl chloride plasticizers, particularly in wire and cable formulations. l,2,4,5-benzene tetracarboxylic acid and 2,6-naphthalene dicarboxylic acid are useful in the production of polyester resins.

EXAMPLE 14 This example illustrates the preparation of terephthalic acid from meta xylene by an embodiment of the process of this invention.

Meta toluic acid is prepared by a known method such as air oxidation of meta xylene in the presence of a cobalt catalyst. 8.72 grams of the potassium salt of meta toluic acid, 5.52 grams of potassium carbonate, 0.72 grams of cadmium iodide, 1.16 grams of basic cupric carbonate, and 2.88 grams of aluminum carbide are charged to a stainless steel reactor under a blanket of C0 The reactor is sealed, heated to 400 Centigrade and held at this temperature for 4 hours. The reactor is cooled to room temperature, opened and its contents analyzed. Results show 21.4 percent of the recovered toluic acid is the para isomer. The para toluic acid is then oxidized to terephthalic acid according to the procedure of Example 2.

EXAMPLE 15 This example illustrates dissolving chromic oxide, produced by the oxidation of an alkyl substituted aryl compound to the corresponding alkali metal aryl carboxylate by an alkali metal dichromate, in strong mineral acid.

To a ml round bottomed flask equipped with an electric heating mantle and reflux condenser are added 10.0 grams of the green, chromic oxide from an oxida tion of p-xylene with Na Cr O,, 1.0 grams chromium trioxide and 100 grams of 75 percent sulfuric acid. The slurry is heated to reflux at about 155 C. The slurry becomes clear after several minutes at this temperature. Refluxing between -160 C is continued for 1 hour. When the solution cools to room temperature, it is put through a fine porosity, fritted glass funnel. No residue is obtained. The chromic sulfate filtrate obtained in this manner, when diluted to the desired sulfuric acid concentration, is used as the anolyte and/or catholyte for electrolytic regeneration of chromic anhydride as shown in the subsequent examples.

EXAMPLE 16 Electrole material: Anode area: Cathode area: Electrode spacing:

Electrolytic grade lead 1 square dicimeter 1 square decimeter inch [/4 inch between the diaphragm and each electrode] Diaphragm material: Polypropylene cloth Electrolyte: Chromic sulfate in 50 percent sulfuric acid such that the solution was 0.697

molar with respect to chromium III. Temperature: 60C Cell voltage: 4.0 4.1 volts Current density: 30-32 amps/square decimeter in starting up the regeneration, the chromic sulfate electrolyte is charged to both the anolyte and the catholyte reservoirs. This electrolyte is circulated through the anode and cathode compartments by pumping means at a rate of approximately 6 feet per second. When the flows are balanced, a potential of 4.0 volts is applied. Samples of the anolyte are then withdrawn periodically to follow the conversion of chromium III to chromium VI (conversion is determined by standard iodimetric thration using sodium thiosulfate). When the conversion reads 38 percent anolyte is continuously withdrawn from the systen. Fresh chromic sulfate electrolyte is added to the circulating catholyte at the same rate that anolyte is being withdrawn. The chromic sulfate electrolyte is added by applying a slight positive pressure to the cathode compartment and thus forcingthe electrolyte through the diaphragm into the anode compartment. The rate of addition and withdrawal is such that a conversion of 38 percent, is maintained. The run duration under these continuous operating conditions is 6 hours. Current efficiency for this regeneration is 79 percent.

EXAMPLE 17 This example illustrates the regeneration of chromic oxide to chromium trioxide in an electrolytic cell at another current density which regeneration comprises Electrode material: Anode area: Cathode area: Electrode spacing:

Electrolytic grade lead 1 square decimeter 1 square decimeter /4 inch [l6 inch between diaphragm and each electrode] Diaphragm material: Polytetrafluoroethylene cloth Electrolyte: Chromic sulfate in 50% sulfuric acid such tyat the solution was l.l57 molar with respect to chromium lll. Temperature: 55-60C Cell Voltage: 4.1-4.6 volts Current density: 20 amps/square decimeter ln starting up the regeneration, the chromic sulfate electrolyte is charged to both the anolyte and catholyte reservoirs. This electrolyte is circulated through the anode and cathode compartments at a rate of approximately 6 feet/second. When the flows are balanced, a potential of 4.1 volts is applied. Samples of the anolyte are withdrawn periodically to follow conversion of chromium III to chromium VI. (Conversion is determined by standard iodimetric titration using sodium thiosulfate.) When the conversion reaches 51%, anolyte is continuously withdrawn from the system. Fresh chromic sulfate electrolyte is added to the circulating anolyte at the same rate that anolyte is being withdrawn. The chromic sulfate electrolyte is added to the anolyte reservoir by means of a metering pump. The rate of addition and withdrawal is such that a conversion of 51 percent is maintained. The run duration under these continuous operating conditions is 6 hours. Current efficiency for the regeneration is 84 percent.

EXAMPLE 18 This example illustrates separating the chromium trioxide from the strong mineral acid solution.

Anolyte from an electrolytic regeneration as illustrated in Examples 16 and 17 is charged to a round bottomed flask fitted with a thermometer, stirrer, condenser and heating mantle. With stirring the temperature is raised to C. About 210 milliliters of water are stripped from 750 milliliters anolyte liquor that has about 45 percent free sulfuric acid. After stripping off the water the flask is cooled to room temperature. The solution is filtered to collect the red, chromic anhydride that crystallizes out from the concentrated sulfuric acid solution. Chromic anhydride is used to acidify the disodium terephthalate solution from the oxidation reaction as in Examples 1 through 14.

EXAMPLE 19 This example illustrates the use of the filtrate, result ing from the acidification of the alkali metal aryl carboxylate by an aqueous solution of chromium trioxide to produce an aryl carboxylic acid as illustrated in Examples 1 through 14, to oxidize an alkyl substituted aryl compound to an aryl carboxylic acid.

A portion of the filtrate from the above described acidification containing 0.2 moles of Na Cr O,-2H O is charged to a 1.4 liter, stainless steel, rocking autoclave reactor along with enough water to make 450 grams total weight. To this is added 7.13 grams (0.0672 mole) p-xylene. The reactor is sealed and heated to 275 C; the rocking reactor is held at this temperature for 2 hours. At the end of this time the reactor is cooled and the reaction mass is filtered. 0.07 grams (0.00066 moles) p-xylene are recovered.

The orange filtrate containing disodium terephthalate and the excess sodium dichromate is acidified. The precipitated terephthalic acid is filtered and washed with water until the washings are colorless. After drying the acid is analyzed and is found to contain 10.43 g (0.0628 moles) terephthalic acid and 0.16 g. (0.0012 moles) p-toluic acid. This corresponds to a 94.6 percent yield of terephthalic acid.

EXAMPLE 20 This example illustrates the use of other preferred alkali metal dichromates in the process of this invention.

The procedure of Example 1 is followed except that in place of sodium dichromate, potassium dichromate is used. The yield of terephthalic acid is substantially the same as in Example 1. An advantage from the use of dipotassium terephthalate is that because of the greater solubility of dipotassium terephthalate in water, reaction vessels used in the described steps of the process of this invention may be sized smaller than when sodium dichromate is used. Smaller sized vessels require less plant space for the process and improve economics of the process by reducing capital investment.

The procedure of Example 1 is followed except that in place of sodium dichromate, lithium dichromate is used. A good yield of terephthalic acid is obtained.

EXAMPLE 21 This example illustrates the use of another preferred strong mineral acid in the process of this invention.

The procedures of Examples 15, 16 and 18 are followed except that in place of sulfuric acid, phosphoric acid is used. Chromic oxide is readily dissolved and oxidized to chromium trioxide. The chromium trioxide is then separated from the phosphoric acid with facility.

EXAMPLE 22 This example illustrates the purification of an aryl carboxylic acid made by the process of this invention.

High yields of a pure acid are obtained by water washing. To a glass lined, 3 liter, stainless steel, rocking autoclave are charged 36.48 grams of terephthalic acid and 1,000 milliliters of water. The autoclave is sealed and heated to 235 C. The autoclave is cooled to 90 C, opened, and the terephthalic acid collected on a filter. The colorless crystals are washed with three, 300 milliliter portions of hot water. The dried terephthalic acid is found to weigh 35.75 g. This represents a yield of 98 percent.

High purity acid is obtained by recrystallization. Trace mineral content is affected by the composition of the purification vessel. To a stainless steel vessel are charged terephthalic acid and distilled water in a weight ratio of 1:13. The terephthalic acid has a 75 ppm chromium content and no detectable iron content. The vessel is sealed and heated to 250 C. After cooling to 90 C the vessel is opened and the contents are discharged onto a filter. The filter cake is washed with hot distilled water. The recrystallized terephthalic acid is found to contain 1.4 ppm chromium and 0.5 ppm iron.

We claim:

1. A process for the preparation of aryl carboxylic acids from corresponding alkyl substituted aryl compounds and partially oxidized alkyl substituted aryl compounds comprising 1 A. reacting an aqueous mixture of an alkali metal dichromate and a feed stock selected from the group which consists of (i) an alkyl substituted aryl compound, (ii) a partially oxidized alkyl substituted aryl compound and (iii) a mixture of (i) and (ii) at a temperature above 150 C whereby said feed stock is oxidized to the corresponding alkali metal aryl carboxylate and said alkali metal dichromate is reduced to chromic oxide,

B. separating said chromic oxide from the reacted aqueous mixture,

C. subjecting a strong mineral acid solution of said chromic oxide to the action of a direct electric current whereby said chromic oxide is oxidized to chromium trioxide,

D. separating said chromium trioxide from said strong mineral acid solution,

E. reacting said chromium trioxide with said reacted aqueous mixture containing said alkali metal aryl carboxylate whereby the corresponding aryl carboxylic acid and an alkali metal dichromate are formed, and

F. separating said aryl carboxylic acid from the aqueous alkali metal dichromate solution.

2. The process of claim 1 wherein the feed stock is a mixture of xylenes and the corresponding aryl carboxylic acid is a mixture of phthalic acids.

3. The process of claim 1 wherein the feed stock is paratoluic acid and the corresponding aryl carboxylic acid is terephthalic acid.

4. The process of claim 1 wherein the alkali metal dichromate is a compound selected from the group which consists of lithium dichromate, sodium dichromate, and potassium dichromate.

5. The process of claim 1 wherein the alkali metal dichromate is sodium dichromate.

6. The process of claim 1 wherein the alkali metal dichromate is potassium dichromate.

7. The process of claim 1 wherein the strong mineral acid is an acid selected from the group which consists of phosphoric acid and sulfuric acid.

8. The process of claim 1 wherein the strong mineral acid is phosphoric acid.

9. The process of claim 1 wherein the strong mineral acid is sulfuric acid.

10. The process of claim 1 wherein said acid solution (step C) is charged to a first compartment of an electrolytic cell which contains a cathode and then passes through a porous membrane into a second compartment of the electrolytic cell which contains an anode and an electric current is applied to said electrolytic cell whereby the chromic oxide is oxidized to chromium trioxide.

11. The process of claim 10 wherein the porous membrane is polypropylene cloth.

12. The process of claim 10 wherein the porous membrane is polytetrafluoroethylene cloth.

13. A process for the preparation of terephthalic acid from para-toluic acid comprising A. reacting an aqueous mixture of para-toluic acid and a sodium or potassium dichromate at a temperature of from about 225 Centigrade to about 300 Centigrade whereby said para-toluic acid is oxidized to disodium or dipotassium terephthalate and said sodium or potassium dichromate is reduced to chromic oxide,

B. separating said chromic oxide from the reacted aqueous mixture,

C. subjecting a sulfuric acid solution of said chromic oxide to a direct electric current whereby said chromic oxide is oxidized to chromium trioxide,

D. separating said chromium trioxide from said sulfuric acid solution,

E. reacting said chromium trioxide with said reacted aqueous mixture containing said disodium or dipotassium terephthalate whereby terephthalic acid and sodium or potassium dichromate are formed, and

F. separating said terephthalic acid from the aqueous sodium or potassium dichromate solution.

14. The process of claim 13 wherein the dichromate is potassium dichromate.

15. In a process for the preparation of terephthalic acid from mixed ortho-, meta-, and para-xylenes wherein the mixed xylenes are oxidized to the corresponding ortho-, meta-, and para-toluic acids, the orthoand meta-toluic acids are isomerized to paratoluic acid and the para-toluic acid is oxidized to terephthalic acid, the improvement which comprises 1. isomerizing orthoand meta-toluic acids to paratoluic acid by 15 16 A. heating. a mixture of orthoand meta-toluic aqueous mixture,

acids to a temperature of from about 150 Cen- C. subjecting a sulfuric acid solution of said tigrade to about 450 Centigrade in the presence chromic oxide to a direct electric current of an alkaline earth metal halide catalyst,a basic whereby said chromic oxide is oxidized to metal carbonate and a disiccant, and chromium trioxide, B. recovering the para-toluic acid, and D. separating said chromium trioxide from said 2. oxidizing said para-toluic acid to terephthalic acid Sulfuric acid Solution,

b E. reacting said chromium trioxide with said aque- A. reacting an aqueous mixture of said recovered 011$ reacted mixture contaifiing Said disfidium para toluic acid and a Sodium dichromate at a terephthalate whereby the terephthalic acid and temperature of from about 225 Centigrade to Sodlum filchrqmate are f about 3 0 Centigrade whereby Said pal-atomic F. separating said terephthalic acid from the 80d!- acid is oxidized to disodium terephthalate and um dlchmmate l said sodium dichromate is reduced to chromic 16. The process of claim 13 wherein the dichromate oxide 15 is sodium dichromate. B. separating said chromic oxide from the reacted 

2. The process of claim 1 wherein the feed stock is a mixture of xylenes and the corresponding aryl carboxylic acid is a mixture of phthalic acids.
 2. oxidizing said para-toluic acid to terephthalic acid by A. reacting an aqueous mixture of said recovered para-toluic acid and a sodium dichromate at a temperature of from about 225* Centigrade to about 300* Centigrade whereby said paratoluic acid is oxidized to disodium terephthalate and said sodium dichromate is reduced to chromic oxide, B. separating said chromic oxide from the reacted aqueous mixture, C. subjecting a sulfuric acid solution of said chromic oxide to a direct electric current whereby said chromic oxide is oxidized to chromium trioxide, D. separating said chromium trioxide from said sulfuric acid solution, E. reacting said chromium trioxide with said aqueous reacted mixture containing said disodium terephthalate whereby the terephthalic acid and sodium dichromate are formed, and F. separating said terephthalic acid from the sodium dichromate solution.
 3. The process of claim 1 wherein the feed stock is paratoluic acid and the corresponding aryl carboxylic acid is terephthalic acid.
 4. The process of claim 1 wherein the alkali metal dichromate is a compound selected from the group which consists of lithium dichromate, sodium dichromate, and potassium dichromate.
 5. The process of claim 1 wherein the alkali metal dichromate is sodium dichromate.
 6. The process of claim 1 wherein the alkali metal dichromate is potassium dichromate.
 7. The process of claim 1 wherein the strong mineral acid is an acid selected from the group which consists of phosphoric acid and sulfuric acid.
 8. The process of claim 1 wherein the strong mineral acid is phosphoric acid.
 9. The process of claim 1 wherein the strong mineral acid is sulfuric acid.
 10. The process of claim 1 wherein said acid solution (step C) is charged to a first compartment of an electrolytic cell which contains a cathode and then passes through a porous membrane into a second compartment of the electrolytic cell which contains an anode and an electric current is applied to said electrolytic cell whereby the chromic oxide is oxidized to chromium trioxide.
 11. The process of claim 10 wherein the porous membrane is polypropylene cloth.
 12. The process of claim 10 wherein the porous membrane is polytetrafluoroethylene cloth.
 13. A process for the preparation of terephthalic acid from para-toluic acid comprising A. reacting an aqueous mixture of para-toluic acid and a sodium or potassium dichromate at a temperature of from about 225* Centigrade to about 300* Centigrade whereby said para-toluic acid is oxidized to disodium or dipotassium terephthalate and said sodium or potassium dichromate is reduced to chromic oxide, B. separating said chromic oxide from the reacted aqueous mixture, C. subjecting a sulfuric acid solution of said chromic oxide to a direct electric current whereby said chromic oxide is oxidized to chromium trioxide, D. separating said chromium trioxide from said sulfuric acid solution, E. reacting said chromium trioxide with said reacted aqueous mixture containing said disodium or dipotassium terephthalate whereby terephthalic acid and sodium or potassium dichromate are formed, and F. separating said terephthalic acid from the aqueous sodium or potassium dichromate solution.
 14. The process of claim 13 wherein the dichromate is potassium dichromate.
 15. In a process for the preparation of terephthalic acid from mixed ortho-, meta-, and para-xylenes wherein the mixed xylenes are oxidized to the corresponding ortho-, meta-, and para-toluic acids, the ortho- and meta-toluic acids are isomerized to para-toluic acid and the para-toluic acid is oxidized to terephthalic acid, the iMprovement which comprises
 16. The process of claim 13 wherein the dichromate is sodium dichromate. 